[Cookbook] [Routing] Article about custom route loaders

I've created a new cookbook article about custom route loaders (see also #526). Feedback is much appreciated.

There is a small difference for version 2.1:

    public function setResolver(LoaderResolver $resolver)

should be changed to 

    public function setResolver(LoaderResolverInterface $resolver)

As well as the corresponding ``use`` at the top.

How should I add this to my pull request?